Research on realizing more shift-stages of an automatic transmission is undertaken to achieve enhancement of fuel consumption and better drivability, and recently, the increase in oil prices has triggered a hard competition in enhancing fuel consumption of a vehicle.
In this sense, research on an engine to achieve weight reduction and to enhance fuel consumption by so-called downsizing research for use with an automatic transmission has been performed to simultaneously provide better drivability and fuel consumption by achieving more shift stages for the automatic transmission.
In order to achieve more shift stages for an automatic transmission, the number of parts is typically increased, which may adversely affect installability, production cost, weight and/or power flow efficiency.
Therefore, in order to maximally enhance fuel consumption of an automatic transmission having more shift stages, it is important for better efficiency to be derived with a smaller number of parts.
In this respect, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages is under investigation.
An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to seven engagement elements (which may be frictional elements), and may easily become lengthy, thereby deteriorating installability.
In this regard, disposing planetary gear sets in parallel, or employing dog clutches instead of wet-type control elements, is sometimes attempted. However, such an arrangement may not be widely applicable, and using dog clutches may easily deteriorate shift-feel.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.